The semiconductor fabrication industry continuously strives to reduce geometries to allow greater numbers of devices to be formed within an integrated circuit (IC). There are competing requirements to form larger masks with finer line widths. It is difficult to increase resolution and increase the field size at the same time.
An illumination beam exposure system includes an illumination optical system to illuminate a mask with an illumination beam, and a projection optical system to project the illumination beam through a mask onto a sensitive substrate. For the critical dimension (CD) to be small, a high degree of controllability is required for the electron beam and pattern placement. In direct write electron beam exposure systems, for any given number of address points in the mask definition, as the spacing decreases with the line width, the size of a writable field decreases commensurately. Further, if the wavelength is shortened to improve resolution, the depth of focus becomes shallower.
To address this issue, it is now common to write a plurality of smaller sub-fields, each within the limits of the electron beam system. A single elongated line may be spread across a plurality of sub-fields. The lines in the individual sub-fields are stitched together, using a step-and-scan exposure system. A small sub-field of the reticle is illuminated, and the scanning stage on which the reticle is positioned is stepped to the location of the next sub-field. The connection ends of the line segments in each sub-field overlap, enabling formation of a fine line having a length that exceeds the size of the largest subfield the beam system is capable of forming. If there are positional errors in registering the corresponding subfields, the result is discontinuities in the line. Two adjacent segments of a line may be offset sufficiently to substantially affect the resistance in the stitching region.
Stitching errors can cause leakage or generate retention time problems, if the narrowest line width in the stitching area is less than 90% of the gate length. Stitching errors can also produce a weaker device in terms of drive current, if the widest line width in the stitching area is too large.
A variety of techniques have been devised to improve the continuity of the stitched lines. For example, U.S. Pat. Nos. 5,055,383 and 5,922,495 describe structures for stitching.
Hiroshi Yamashita et al., “Recent Progress in Electron-Beam Cell Projection Technology” Jpn J. Appl. Phys., Vol. 35 (1996) pp. 6404–6414 describes patterns that can be used in the overlap regions, including partially overlapping convex “V” shaped line segment ends. Another embodiment is described having a pair of complementary convex and concave “V” shaped ends.
FIG. 1A shows an example of stitching using Yamashita's additional patterns. A pair of adjacent line segments 100, each have a main pattern area 103 and convex “V” shaped connecting ends 101, which overlap in the stitching area 102. FIG. 1B shows the dosage profile for the stitched line of FIG. 1A. The main portion 103 of each line 100 has a constant dosage 110. At the convex ends 101, the dosage level 111 continuously drops off from the full dosage 110 to zero. In the stitching region 102, the total dosage 112 from the two overlapping patterns is approximately the value of the full dosage 110 in the main segments 103 of the lines.
Horiuchi, Toshiyuki, “Gradation Stitching Exposure for Step-and-Scan Projection Printing System,” Jpn J. Appl. Phys. Vol. 37 (1998) pp. 6641–6647 describes a technique for using the electron beam to provide a variable exposure dose to the connection ends of each line segment in the stitching area. Horiuchi's step-and-scan apparatus includes a blind slit having a convex pattern (e.g., “V” shaped) on the end of the segments, so that the pattern formed has a convex pattern in the stitching area. The overlapping length is set large relative to the line width, and the taper of the “V” shape is gentle, so that the even in the presence of stitching error (offset) in the width direction, the gradation slope of the dose is not excessive.
In Horiuchi's method, the doses change continuously at the boundary of the stitched fields, falling off to substantially zero dosage at the very end of each segment. The convex patterns are generally configured so that, if there are no stitching errors (positional misalignment in the length or width directions), the dosage in the overlapping stitching region is approximately constant.